Fuel cells are cells that utilize hydrogen and oxygen to generate direct-current power, and are broadly divided into a solid electrolyte type, a molten carbonate type, a phosphoric acid type and a solid polymer type. The respective types derive from the constituent material of an electrolyte portion that constitutes the basic portion of the fuel cell.
Currently, fuel cells that have reached the commercial stage include phosphoric acid type fuel cells that operate around 200° C., and molten carbonate type fuel cells that operate around 650° C. With the progress of technological development in recent years, solid polymer type fuel cells that operate around room temperature and solid electrolyte type fuel cells that operate at 700° C. or more are attracting attention as small-size power sources for mounting in automobiles or for household use.
FIG. 1 is a multiple-view explanatory drawing that illustrates the structure of a solid polymer fuel cell, in which FIG. 1(a) is an exploded view of a fuel cell (unit cell), and FIG. 1(b) is a perspective view of the entire fuel cell stack.
As illustrated in FIG. 1(a) and FIG. 1(b), a fuel cell stack 1 is an assembly of unit cells. As shown in FIG. 1(a), each unit cell has a structure in which a fuel electrode layer (anode) 3 is laminated on one surface of a solid polymer membrane 2, an oxide electrode layer (cathode) 4 is laminated on the other surface, and separators 5a and 5b are overlaid on the two surfaces.
A fluorine ion exchange resin film that has hydrogen ion (proton) exchange groups is a typical example of the solid polymer membrane 2.
The fuel electrode layer 3 and the oxide electrode layer 4 respectively include a diffusion layer, and a catalyst layer provided on a surface on the solid polymer membrane 2 side of the diffusion layer. The diffusion layer is composed of carbon paper or carbon cloth constituted by carbon fiber. The catalyst layer is composed of a particulate platinum catalyst, graphite powder, and a fluorocarbon resin having hydrogen ion (proton) exchange groups. The catalyst layers of the fuel electrode layer 3 and the oxide electrode layer 4 come in contact with fuel gas or oxidizing gas that permeates through the diffusion layer, respectively.
A fuel gas (hydrogen or a hydrogen-containing gas) A is fed through channels 6a provided in the separator 5a to supply hydrogen to the fuel electrode layer 3. An oxidizing gas B such as air is fed through channels 6b provided in the separator 5b to supply oxygen. The supply of these gases causes an electrochemical reaction, to thereby generate direct-current power.
The functions required of a solid polymer fuel cell separator are: (1) a function as a “channel” for supplying a fuel gas with in-plane uniformity on the fuel electrode side; (2) a function as a “channel” for efficiently discharging water produced on the cathode side from the fuel cell system together with carrier gases such as air and oxygen after the reaction; (3) a function as an electrical “connector” between unit cells that maintains low electrical contact resistance and favorable electric conductivity as an electrode over a long time period; and (4) a function as an “isolating wall” between adjacent cells for isolating an anode chamber of one cell from a cathode chamber of an adjacent cell.
Research of various kinds has been conducted up to now concerning base materials for separators that exert the foregoing functions. The materials used for separators are broadly classified into metal materials and carbon-based materials.
With regard to carbon-based materials, the application thereof to a separator composed of a carbon plate material has been earnestly studied at the laboratory level. However, there is a problem in that a carbon plate material easily cracks, and furthermore there is a problem in that machining costs for making the surface even and machining costs for gas channel formation are extremely high. Each of these is a significant problem, and makes the commercialization of fuel cells itself difficult.
There is a movement towards attempting to apply a carbon composite material that adopts a thermoplastic resin or a thermosetting resin as a binder, instead of a carbon plate material. Powders of flake graphite, lumpy graphite, acetylene black, carbon black, Ketjen black, expanded graphite and artificial graphite and the like are used as electroconductive carbonaceous powders, and powder having a mean particle diameter in a range of around 10 nm to 100 μm is used. The development of resins for use as a binder is being actively conducted, and recent improvements in performance are noticeable and there are also remarkable improvements in productivity and costs.
For example, Patent Document 1 discloses a separator for a polymer electrolyte fuel cell that is interposed between gas diffusion electrodes of the fuel cell, and that has, on one side or on both sides of a carbon composite material, a groove for supplying an oxidizing agent gas or a fuel gas, the carbon composite material consisting of an expanded graphite powder and a thermoplastic resin or a thermosetting resin or a fired product thereof, with the expanded graphite powder being dispersed in the thermoplastic resin, the thermosetting resin or the fired product thereof and the expanded graphite powder having a mean particle diameter of 5-12 μm, and at least 80% of the total particles of the expanded graphite powder having particle diameters in a range of 0.1-20 μm.
In Patent Document 1, the reason for defining the mean particle diameter is described as follows. In a case where the mean particle diameter of the expanded graphite is less than 5 μm, it is difficult for the thermoplastic resin or thermosetting resin to permeate between particles of the expanded graphite. Therefore, there is a significant loss in the gas barrier property. Conversely, in a case where the mean particle diameter is greater than 12 μm, it is difficult for the thermoplastic resin or thermosetting resin to fill-in gaps between particles of the expanded graphite. As a result, not only is there a significant loss in the gas barrier property, but the packing density falls and the electrical connection is insufficient, and consequently the electrical conductivity decreases.
Patent Document 2 discloses a fuel cell separator that is formed of a base material which is composed of at least a binder, a powdered carbon filler having a mean particle diameter of 10 nm to 100 μm, and short fibers having a mean fiber length of 0.07 to 3.0 mm, and which is obtained when the quantity ratio between these components is 200 to 800 parts by weight of the powdered carbon filler and 68 to 300 parts by weight of the short fibers with respect to 100 parts by weight of the binder, and in which a bending deflection in accordance with JIS K 6911 is from 0.5 to 1.0 mm.
Patent Document 3 discloses that, to make a separator substrate for a fuel cell that is excellent in impact resistance and toughness, a rubber-modified phenol resin is used as a binder in a separator substrate for a fuel cell that is formed by molding a mixture that includes at least electrically conductive powder and a binder.
The proportion of the rubber-modified phenol resin is given as 5 to 50 parts by weight with respect to 100 parts by weight of the electrically conductive powder. The bending modulus is given as 40 to 1 GPa, and the deflection at the time of rupturing when performing a bending test is given as 0.1 to 3 mm. It is stated that the mean particle diameter of the electrically conductive powder is 10 nm to 100 μm, preferably, 3 μm to 80 μm, and that formability can be improved when the mean particle diameter is 10 nm or more, and that the electrical conductivity can be improved when the mean particle diameter is 100 μm or less.
The “rubber-modified phenol resin” can be obtained by reacting an unvulcanized rubber with a phenol resin. As examples of the unvulcanized rubber, there can be mentioned one kind or a mixture of two or more kinds selected from the group consisting of fluorocarbon rubber, silicone rubber, butyl rubber, chloroprene rubber, nitrile rubber, nitrile-chloroprene rubber, chlorinated butyl rubber, chlorinated polyethylene, epichlorohydrin rubber, epichlorohydrin-ethylene oxide rubber, epichlorohydrin-ethylene oxide-acrylic glycidyl ether terpolymer, urethane rubber, acrylic rubber, ethylene-propylene rubber, styrene rubber, butadiene rubber and natural rubber.
Patent Document 4 discloses a fuel cell separator that is formed by molding a composition for a fuel cell separator that is composed of graphite, an epoxy resin, a polycarbodiimide resin as a curing agent, a curing accelerator and a mold release agent, in which the mean particle diameter of the graphite is 50 to 500 μm, and 10 parts by mass or less of the epoxy resin, 9 parts by mass or less of the polycarbodiimide resin, 0.3 parts by mass or less of the curing accelerator and 0.5 to 3 parts by mass of the mold release agent are added with respect to 100 parts by mass of graphite.
Patent Document 5 discloses a separator for a fuel cell that includes an electrically conductive core portion formed of a metal material or a metal composite material, an electrically conductive adhesive layer that covers the electrically conductive core portion, and an electrically conductive skin portion formed on the electrically conductive adhesive layer, in which the electrically conductive core portion and the electrically conductive skin portion are bonded together by the electrically conductive adhesive layer.
The electrically conductive adhesive layer is formed of a carbon-containing conductive adhesive that adopts two or more kinds of carbon powder as an electrically conductive filler and adopts a resin as a binder and contains 10 to 67 parts by weight of the resin with respect to 100 parts by weight of the electrically conductive filler. The electrically conductive skin portion is formed of a carbon-containing composite material that adopts a carbon powder as an electrically conductive filler and adopts a resin as a binder and contains 3 to 20 parts by weight of the resin with respect to 100 parts by weight of the electrically conductive filler.
Titanium, aluminum, stainless steel or the like, specifically, for example, an aluminum plate or a stainless steel plate, or a metal composite material obtained by coating a noble metal or a carbon material of the aforementioned metal materials are exemplified as the metal material constituting the electrically conductive core portion, and it is stated that, in order to enhance the adhesiveness with respect to the electrically conductive skin portion, it is also favorable to perform a surface treatment by a mechanical polishing method such as a blasting treatment, a discharge treatment, lapping or polishing.
Patent Document 6 discloses a method for manufacturing a fuel cell separator according to which a composition for a fuel cell separator mainly containing a conductive material, a binder, and an additive is mixed, granulated and dried, and thereafter a granulated substance obtained by the granulating is packed in a die and subjected to hot-press molding, in which the mean particle diameter of the aforementioned granulated substance is 60 to 160 μm and the granulated substance has the particle size distribution described hereunder, and the residual volatile matter content of the granulated substance is 4% by mass or less.
Particle Diameter Percentage
5 μm or more and less than 100 μm: 10 to 80%
100 μm or more and less than 300 μm: 10 to 40%
300 μm or more and less than 500 μm: balance
However, high-temperature durability and hydrolytic durability still remain as problems with respect to carbon composite materials, and the problem of deterioration over time in an organic resin for binding that occurs during application to a fuel cell is a significant issue. Further, problems such as responding to increasingly stringent demands for dimensional accuracy and wall thinning, carbon corrosion that progresses under the influence of cell operation conditions, and unexpected cracking problems that arise when a fuel cell is assembled and during use thereof remain as issues to be solved in the future.
On the other hand, as a metal material, numerous studies have been made regarding the use of stainless steel for a separator. In addition, in order to solve the problems associated with carbon-based materials, the development of a carbon-based separator that includes a core material (core) composed of a metal material such as stainless steel and that has a carbon layer having electrical conductivity which is formed on the surface of the core material is also progressing.
For example, Patent Document 7 discloses stainless steel suitable as a separator of a solid-oxide fuel cell. Further, Patent Documents 8 and 9 disclose a solid polymer fuel cell that includes a separator made from ferritic stainless steel.
Patent Document 10 discloses ferritic stainless steel for a separator of a solid polymer fuel cell, and a solid polymer fuel cell using the same in which the ferritic stainless steel contains 0.01 to 0.15 mass % of C and in which Cr carbides precipitated.
Patent Document 11 discloses stainless steel for a separator of a solid polymer fuel cell in which one or more kinds among M23C6 type, M4C type, M2C type and MC type carbide-based metal inclusions and M2B type boride-based metal inclusions which have electrical conductivity are dispersed and exposed on the stainless steel surface, and states a ferritic stainless steel that contains, by mass %, C: 0.15% or less, Si: 0.01 to 1.5%, Mn: 0.01 to 1.5%, P: 0.04% or less, S: 0.01% or less, Cr: 15 to 36%, Al: 0.001 to 6% and N: 0.035% or less, in which the respective contents of Cr, Mo and B satisfy the expression 17%≤Cr+3×Mo−2.5×B, with the balance being Fe and unavoidable impurities.
Patent Document 12 describes a method for producing stainless steel material for a separator of a solid polymer fuel cell in which the surface of the stainless steel material is etched using an acidic aqueous solution to expose one or more kinds among M23C6 type, Mat type, M2C type and MC type carbide-based metal inclusions and M2B type boride-based metal inclusions having electrical conductivity on the surface, and also discloses a ferritic stainless steel material that contains, by mass %, C: 0.15% or less, Si: 0.01 to 1.5%, Mn: 0.01 to 1.5%, P: 0.04% or less, S: 0.01% or less, Cr: 15 to 36%, Al: 0.001 to 1%, B: 0 to 3.5%, N: 0.035% or less, Ni: 0 to 5%, Mo: 0 to 7%, Cu: 0 to 1%, Ti: 0 to 25×(C %+N %), and Nb: 0 to 25×(C %+N %), in which the respective contents of Cr, Mo and B satisfy the expression 17%≤Cr+3×Mo−2.5×B, with the balance being Fe and impurities.
Further, Patent Document 13 describes a solid polymer fuel cell in which an M2B type boride-based metal compound is exposed on the surface, and when an anode area and a cathode area are taken as 1, respectively, a proportion of the area of the anode which directly contacts a separator and a proportion of the area of the cathode which directly contacts a separator are each within a range of 0.3 to 0.7, and also discloses stainless steel in which one or more kinds among M23C6 type, M4C type, M2C type and MC type carbide-based metal inclusions and M2B type boride-based metal inclusions which have electrical conductivity are exposed on the stainless steel surface.
In addition, Patent Document 13 discloses, with respect to the stainless steel constituting the separator, a ferritic stainless steel material is disclosed that consists of, by mass %, C: 0.15% or less, Si: 0.01 to 1.5%, Mn: 0.01 to 1.5%, P: 0.04% or less, S: 0.01% or less, Cr: 15 to 36%, Al: 0.2% or less, B: 3.5% or less (however, excluding 0%), N: 0.035% or less, Ni: 5% or less, Mo: 7% or less, W: 4% or less, V: 0.2% or less, Cu: 1% or less, Ti: 25×(C %+N %) or less and Nb: 25×(C %+N %) or less, in which the respective contents of Cr, Mo and B satisfy the expression 17%≤Cr+3×Mo−2.5×B.